I have a new macbook air with 10.8 Mountain Lion, a standard router and is planning on buying a speaker (Nad Viso 1) and need an airport express to get it wireless.

Would i be fine if i went with the A1264 model? Or would you definitely recommend getting the newest A1392 model?

I do not need the extra Ethernet port at all. I just want to be as future proof as possible without spending extra money on something i do not need. What i am afraid of, is of course then mavericks is out, then i am already left behind, or do you think that the A1264 version would work with Mavericks?

For your purposes, I would get the older one. IMO it is better because there is no power cord to deal with. It is just a standard 802.11b/g/n router and will work with Mavericks or anything else for years to come.